1. Field of the Invention
The present invention relates to a passive and active type focus detecting system of a high accuracy having plural sets of apertures capable of adjusting a focus detecting position in consideration of aberration of a photographic lens mounted on a camera body at the time of focus detection.
2. Description of the Prior Art
Focus detecting methods in automatic focus detecting systems for camera are classified into a passive type focus detecting method (hereinafter referred to as "passive AF") in which focus detection is performed using an external light, and an active type focus detecting method (hereinafter referred to as "active AF") in which a focus detecting infrared light is directed to an object from the camera side and the reflected light is detected for focus detection. The latter, i.e., the active AF, includes a TTL active type focus detecting method (hereinafter referred to as the "TTL active AF") in which a light source is incorporated in a camera, light from the light source directed to an object through a photographic lens, and the reflected light is conducted to a light sensing element again through the photographic lens and is detected.
In the TTL active AF it is necessary to take measures to prevent radiated light from being reflected between surfaces of constituent lens elements of a photographic lens and becoming incident as a disturbing light on the light sensing element. In an interchangeable lens type camera system, there are provided a variety of interchangeable lenses having different focal lengths and open aperture values, so the focus detecting system used therein is also required to permit the use of those many kinds of interchangeable lenses.
To satisfy the requirement just mentioned above it is necessary to adopt a construction in which a light sensing element for focus detection is disposed so as to detect light passing through a region close to a lens optical axis and thereby permit the use of a photographic lens having a large open aperture value. However, such an arrangement permits easier influence of the aforementioned disturbing light upon the light sensing element.
As a countermeasure there has been proposed a construction (see Japanese Laid Open Patent Publication No. 57-22210) in which an optical axis of a projection optical system and that of a light sensing optical system for focus detection are disposed so as not to be in point symmetry with respect to an optical axis of a photographic lens on a main plane of the same lens. However, as long as the optical axis of the projection optical system and that of the light sensing optical system for focus detection are disposed in the vicinity of the optical axis of the photographic lens, it is very difficult to completely eliminate the influence of the foregoing disturbing light.
The following concrete description is now provided about in what manner such disturbing light is generated.
FIG. 1 shows an example of a focus detecting optical system, in which L.sub.0 and F denote a photographic lens and a predetermined focal plane, respectively, and a focus detecting optical system A.sub.0 is disposed behind the focal plane F. P and Q denote masks disposed in front of lenses. And the mark C denotes a light sensing element for focus detection, which is a CCD line sensor for example.
FIG. 2(a) and FIG. 2(b) are explanatory views showing in what manner the disturbing light is generated, in which a beam of light radiated from point O positioned on a predetermined focal plane on an optical axis of a photographic lens is reflected by the surfaces of each lens which constitute the photographic lens. FIG. 2(a) typically shows only once-reflected light beams reflected by surface Nos. 1, 2 and 6 of lenses and returning in the incident direction, while FIG. 2(b) shows thrice-reflected light beams reflected by surface Nos. 6, 1 and again 6 and returning in the incident direction.
FIG. 3(a) shows spreads of disturbing light beams on the plane of the first mask Q in the focus detecting optical system shown in FIG. 1, the disturbing light beams spreading in point symmetry ranges with respect to the optical axis of the photographic lens. The circles in this figure represent regions of disturbing light beams reflected by the surfaces of constituent lenses of the photographic lens. Although only once-reflected disturbing light beams are here shown, these are also the cases with thrice or more reflected light beams.
FIG. 3(b) shows spreads of disturbing light beams on the plane of the second mask P in the focus detecting optical system shown in FIG. 1, the spread being smaller by an amount restricted by the first mask Q.
FIG. 4 shows how disturbing light beams vary according to different types of photographic lenses, in which there are illustrated spreads of disturbing light beams on the plane of the second mask P in the focus detecting optical system shown in FIG. 1 like that illustrated in FIG. 3(b). In FIG. 4, the upper stage is with the photographic lens in an infinite distance position, while the lower stage is with the same lens in the closest distance position. In the transverse direction there are shown a lens having a focal distance of 50 mm and an open aperture value of 1.7 as well as two different zooming conditions L and S of a zoom lens of 35-105 mm in focal length and 3.5-4.5 in open aperture value. That the spread of disturbing light beam changes with zooming is because of a positional change of lens elements which constitute the zooming lens component. The spread of disturbing light beam also changes with a focusing condition of the photographic lens.
Thus, when an aperture mask P in the focus detecting optical system is disposed near the optical axis of the photographic lens, the degree of influence of disturbing light beam changes according to the kind of the photographic lens, the focusing condition thereof, and the zooming condition thereof. Therefore, the detection of an exact in-focus position has not always been detected by the TTL active AF.